The present invention concerns a system and method for thermoregulations of a living body. The system and method of the invention may be applied to both humans and animals.
A living body can maintain a normal body temperature, which in humans is about 36.5-37xc2x0 C., under normal conditions. However, when subjected to extreme temperature conditions or to a variety of medical treatments, the body may not be able to properly regulate the body temperature. This is the case, for example: in individuals exposed to extreme temperatures; and in a variety of physiological disorders and medical conditions (e.g. general anesthesia) where the body loses its ability to properly regulate the body temperature.
Hypothermia is a reduction in body temperature which may result from exposure to a cold environment. Mild hypothermia in which body""s core temperature (namely the temperature of internal organs and tissue within the body) is reduced by about 2xc2x0 C. below the normal temperature, commonly occurs during and following surgery that is conducted under general anesthesia, as patients tend to lose body heat due to lowered metabolism and as a result of exposure of internal body organs or tissue, e.g. in an abdominal or thoracic cavity, to ambient air.
Hypothermia is experienced by more than 60% of patients after surgery. and in some surgical procedures, e.g. open heart surgery, hypothermia is experienced by more than 90% of adult patients.
Hypothermia can cause various severe physiological problems including cardiac morbidity, coagulopathy, impaired immune function, impaired wound healing, shivering, altered pharmaconnected properties of drugs, etc.
Hyperthermia, namely increase in body temperature above normal levels, may also cause various physiological disorders and if extreme, may be life threatening.
The clinical need to provide means for body temperature control has long been recognized.
MicroClimate Systems Inc., Stanford, Mich., U.S.A. markets a series of portable personal cooling systems (sold under the trademarks KOOLVEST, KOOLJACKET, KOOLBAND, KOOLPAID and others) which are intended for use by healthy persons who perform a physical activity in a hot environment (see Internet http://www.microclimate.com/prodline.html or http://www.microclimate.com/work.html). The system consists of a garment (it may be a vest, jacket, skull cap, a cervical collar, etc.) with tubing embedded therein in which water flows propelled by a battery-powered pump. Water passes through ice or through a cooling device before entering the tubing within the garment. The user can control the pump rate, the temperature, etc. A similar product line is also provided by Mallinckrodt Inc., St. Louis, Mo., U.S.A. (see the Internet at http://www/mallinckrodt.com/ccd) and others.
A similar system is disclosed in U.S. Pat. No. 4,807,447 (Macdonald et al). The unique feature of this system is that the person""s breathing is used to operate the refrigerant compressor.
Various systems making use of garments, pads or blankets with heat transfer elements, e.g. conduits embedded therein, are disclosed in U.S. Pat. Nos. 3,738,367, 4,094,357, 4,094,367, 4,149,541, 4,844,072, 5,184,612 and 5,269,369. However, due to various physiological reasons, realized only in accordance with the present invention and which will be specified further below, none of the above systems provides adequate control of body temperature of the individuals in general, and of patients following surgery before undertaking general anesthesia in particular.
There is also a need for control of body temperature in veterinary medicine, particularly the control of body temperature during surgery under general anesthesia. Animals, and particularly small animals, occasionally die following surgery, owing to uncontrolled changes in body temperature.
In the following, the term xe2x80x9ccore temperaturexe2x80x9d will be used to denote the temperature within the body, namely that of the internal organs and tissue. Core temperature is typically measured through the rectum (xe2x80x9crectal temperature measurementxe2x80x9d) but may also he measured by inserting probes through a variety of other body cavities, e.g. mouth, nasal or ear temperature probes. The term xe2x80x9csurface temperaturexe2x80x9d will be used to denote the temperature of the external body surface, (which may be that of the skin or, where the skin has been removed, e.g. in a bun injury, that of the most external layers). It should be noted that surface temperature may vary between different body portions. The surface temperature may be measured by a variety of temperature probes including, for example, an infrared sensor measuring infrared emission from a specific skin portion, probes attached to the skin such as a thermo-couple devices, thermistor, etc.
In accordance with the present invention, it has been realized that a major deficiency of prior art thermoregulatory devices is that these do not sufficiently reconcile the fact that the body is not a passive heat conductor but rather employs intrinsic dynamic physiological thermoregulation mechanisms which can change the heat transfer properties between the body""s core and the periphery. For example, when exposed to a low temperature, there is a vasoconstriction of peripheral blood vessels, thus reducing blood flow to the periphery and thereby reducing heat transfer between the periphery and the body""s core. In other situations there is vasodilatation resulting in increased blood flow to the periphery above normal levels thus bringing an increase in heat transfer rate between the body""s core and the periphery.
In accordance with the invention, a system and method for the control of body temperature is provided. The system and method of the invention allows controlled cooling of body temperature, controlled heating, as well as regulating an individual""s body temperature to remain at a desired set temperature. It is a unique feature of the invention that it takes into consideration the complexity of the heat transfer regiment from the skin to the body""s core. This complexity is manifested by dynamic heat transfer properties, namely physiological changes which can alter the rate in which heat can be transferred between the skin and the body""s core. Such heat transfer dynamics (HTD) may be determined by a variety of means, as will be illustrated further below.
In accordance with the invention there is provided a system for controlling an individual""s body core temperature (BCT) comprising:
a heat exchanger for transferring heat to or removing heat from portions of the individual""s body surface;
at least one BCT-sensing device for measuring the individual""s actual BCT (aBCT) and emitting an aBCT data signal;
at least one sensing device for measuring a parameter indicative of the heat transfer dynamics (HTD) between the body surface and the body""s core, and emitting an HTD data signal; and
a control module for receiving data signals from measuring devices. comprising the aBCT data signal and the HTD data signal, and for emitting a control signal for controlling heat exchange properties of said heat exchanger as a function of the data signals and the desired body core temperature (dBCT).
The heat exchange may either be provided with an internal heat or cold producing capability, e.g. including a Peltier effects modules, or the heat exchanger may be linked to at least one source of cold and/or hot fluid, which fluid then circulates between such source and the heat exchanges to transfer heat/cold between the exchanger and said source.
By another aspect, the present invention provides a method for controlling internal body temperature of an individual, comprising the following steps:
(a) defining a desired body core temperature (dBCT);
(b) providing a heat exchanger and bringing it into contact with at least a portion of the individual""s external body surface, for heat transfer to or heat removal from the body portion;
(c) continuously measuring parameters including at least the actual body core temperature (aBCT) and a parameter indicative of the heat transfer dynamics (HTD) between the body""s surface and the body"" core; and
(d) in a processor, receiving data signals corresponding with the measured parameters, comparing the aBCT with the dBCT, defining based thereon whether there is a need for heating or cooling and emitting a control signal controlling heat transfer properties of said heat exchanger based on the aBCT/dBCT difference and on said HTD.
The control of the heat exchange properties of said heat exchanger may involve change of the heat transfer properties between the heat exchanger and the body surface which may be achieved, for example, by changing the heat conductance parameters between the body""s surface and the skin, e.g. by pumping or removing air into or from air pockets disposed between heat radiating/heat absorbing members within the heat exchanger and the skin; or preferably, by changing the temperature of the heat exchanger, which may either be a reduction in the extent of heating or cooling, halting the heating or cooling operation, or reversing the heating or cooling operation into cooling or heating, respectively. In reversing, the heat exchanger acting first as a heat source will be switched to become a heat sink, or vice versa, thus reversing the direction of heat transfer.
The heat exchanger may, for example, comprise electric heating/cooling devices, e.g. Peltier devices and others. However, in accordance with a preferred, non-limiting, embodiment of the invention, the heat exchanger is of a kind comprising one or more conduits or fluid transfer medium for passing the heat control fluid therethrough, for transferring heat to or absorbing heat from the individual""s body surface. The fluid, which is typically, though not exclusively, a liquid, e.g. water. may be driven through the conduits or medium by a pump or any other suitable device therefor. Such fluid thus circulates between the heat exchanger and a heat and/or cold source. The heat exchanger is typically flexible to allow it intimate contact with a body surface for efficient heat transfer therewith.
In addition to the above noted measuring devices (the BCT sensing device and the device for measuring a parameter indicative of said HTD), the system may further comprise one or more devices for measuring temperature of the circulating fluid and for emitting data signal relating thereto to the controller. At times, where the system comprises two or more such devices, at least one of which may serve as an inlet temperature sensing device for measuring temperature of the fluid as it enters the at least one conduit or fluid transfer medium, and at least one other may serve as an outlet temperature sensing device for measuring temperature of the fluid as it exits the at least one conduit or fluid transfer medium. The temperature drop (xcex94T) between the garment""s inlet and the outlet is a very good indicator of said HTD, since this information, together with information on the fluid""s flow rate, permits an accurate calculation of the heat transfer between the heat exchanger and the body, which depends on said HTD. Thus, in accordance with a preferred embodiment said xcex94T and the fluid flowrate are used as an HTD-indicating parameter.
The heat exchanger of the invention is typically a garment which is worn over a portion of the individual""s body. Typically, the garment may be designed so as to cover at least about 30%, preferably at least about 40% of the body""s surface. In this way, the system of the invention effectively stabilizes an individual""s body temperature, at a desired body core temperature, within a minimal tolerance. Such a garment may have different forms depending on its intended use. For example, the garment may be designed for use by individuals performing hard labor under extreme hot conditions, for the purpose of cooling their body. For such a purpose the garment has to be relatively flexible so as to limit, as little as possible, the free movement of the individual. By another example, the garment may be designed for medical use, either for the purpose of reducing core temperature of individuals having a disease with a very high fever, for the purpose of increasing core body temperature of hypothermic individuals or for both. A currently preferred embodiment of the invention is the application of the system for control of body temperature of patients during or after general anesthesia. For this purpose the heat exchanger, typically in the form of a garment, may have a variety of openings permitting access for the performance of various surgical procedures, for parental administration of drugs or fluids or for drainage of body fluids, (e.g. excretions or blood).
One example of the use of the system and method of the invention in accordance with the above referred embodiment, is in open heart surgery. Open heart surgery typically involves cooling of the body temperature to about 32xc2x0 C. During such surgery the heart""s activity is temporarily arrested and the blood is circulated through an artificial heart-lung apparatus. Following such surgery, there is a need to increase the body temperature to normal temperature as rapidly as possible, since maintaining such a core body temperature for prolonged time periods may be damaging, at times even life threatening. Today""s medical practice involves, towards the end of the surgery, heating of the blood as it circulates through the artificial heart-lung apparatus. However, this heating practice has proven insufficient, as the body""s core is not sufficiently heated in this way, and once the blood is disconnected from this apparatus and allowed to recirculate through the reactivated heart, the blood temperature immediately drops. Heating in such procedures is typically performed today by the use of heated blankets, blowing of hot air on the individual, etc. Overall, the methods available today do not allow an efficient monitoring and control of body core temperature. In accordance with the invention, the body core temperature may be controllably cooled during the initial phases of the surgery, then maintained at about 32xc2x0 C. while applying heating or cooling as needed, and then eventually controllably heated at the end of the surgery to reach normal body temperature.
Generally, the system and method of the invention may be used in control of patient""s body temperature during the perioperative period in a wide range of different surgeries.
As will no doubt be appreciated, a heat exchanger in the form of a garment may typically be designed to have various forms and sizes, to meet specifications of individuals of various ages, weights, agendas, etc., or to meet specific requirements of a specific medical procedure.
In addition to use for humans, particularly in human medicine, the method and system of the invention may also find a wide range of applications in veterinary medicine. The same problems of resuming normal body temperature, as well as a controlled cooling during surgery, are also encountered in veterinary medicine. As will be appreciated, the heat exchanger, typically in the form of a pad, a blanket or garment, may be designed to have different shapes, depending on the type of animal, the type of surgery, etc.
A system of the invention for use in the operation room, may typically be designed for exchange of data with other instruments present in the operation room, e.g. import of data relating to blood temperature from heart-lung apparatus, import of data relating to heartbeat or blood pressure from respective measuring devices, etc.
The sensing device for measuring a skin parameter indicative of said HTD (hereinafter referred to at times as xe2x80x9cHTD device xe2x80x9d), may, in accordance with the one embodiment, include a device for measuring a temperature at a skin portion proximal to a skin portion on which the heat exchanger is applied. The HTD may then be determined, for example, by either one or both of
(i) determining the rate of temperature change at said skin portion following heating or cooling of adjacent skin portions by the heat exchanger, or
(ii) by assessing the rate of change of temperature difference between the skin portion and the core during heating or cooling of the body.
Said HTD device may, in accordance with another embodiment of the invention, consist of the aforementioned at least two sensing devices for measuring temperature of the fluid as it enters the at least one conduit in the heat exchanger and the temperature as it exits from the at least one conduit. The controller, thus receiving at least two data signals relating to the measured temperature, then calculates said HTD based on the inlets or outlet temperature differential and on the fluid flow rate, which is either determined by the controls or measured by an appropriate measuring device.
In accordance with other embodiments, said HTD device is adapted for measuring a parameter indicative of said HTD, which parameter may be one of a variety of skin and peripheral blood flow parameters. These may be determined by many techniques, e.g. by echo doppler signal techniques, skin conductance, peripheral blood pressure, skin temperature, skin color, etc.
One major feature of the invention relates to the measurements of said heat transfer dynamics (HTD), and taking the heat transfer dynamics into consideration in the heat control regime of the individual. Specifically. when the HTD parameters point to the occurrence of vasoconstriction, any applied cooling should be temporarily halted or reduced. At times, it is advantageous also to reverse the heat transfer mode, temporarily heat in a cooling mode. This means that a cooling mode will involve occasional heat pulses timed and patterned according to said HTD.
The system may have a user interface permitting a user to enter a dBCT, namely a temperature set point of the system. The user interface may further comprise control means allowing selective operation of the system in either an automatic mode, namely in a mode permitting both cooling or heating depending on the direction or deviation of the aBCT from the dBCT. In addition, the control means may also typically allow to select a heat only mode or a cool only mode.
Typically, the heating will be limited so that the temperature at the surface of the heat exchanger which is in touch with the body surface, will not exceed maximum temperature, e.g. a temperature of about 40xc2x0 C. and not to fall below a minimum temperature, e.g. about 15xc2x0 C.
In order to be effective in cooling or heating, the heat exchanger, typically in the form of a garment, has to be fitted onto the individual""s skin. At times, there is a need to wear such a garment for a prolonged period of time, and this may give rise to a risk of pressure wounds. In order to circumvent this problem in accordance with one embodiment of the invention, the heat exchanger has two or more individually flow-controlled flow sub-systems, and these sub-systems may then be used intermittently, namely, one system being inflated with fluid and used, while the other being deflated and thus not exerting pressure on the skin; and vice versa. In accordance with another embodiment, the fluid transfer to the garment is temporarily halted for periods of several seconds to minutes in order to reduce the pressure onto the skin thus reducing currents of pressure.
In accordance with one embodiment of the invention, the system comprises an electric in-line fluid heating/cooling unit and the circulation fluid is directed to flow through said unit for heating or cooling. Heating or cooling of the fluid in such a unit may also be achieved by means of an auxiliary circulatory heat transfer fluid through the intermediary of a heat exchanger within said unit. In accordance with another embodiment, the system comprises at least one cold fluid reservoir and at least one hot fluid reservoir and comprises a fluid flow control system for selectably drawing fluid from these reservoirs. One advantage of having independent hot and cold fluid reservoirs,. is that the switching between heating and cooling modes can be rapid.
In a system comprising independent hot fluid and cold fluid reservoirs, the flow control system is preferably adapted to permit return fluid to flow back into the reservoir from which it was drawn. It is preferred that during switching from a cold to hot fluid or vice versa, the original fluid will flow initially to the reservoir from which it was drawn, and only after the warm fluid has been exploited will the returned fluid be directed to the other reservoir. Otherwise, the cold reservoir may be heated or the hot reservoir cooled. This may be achieved by having a temperature sensing device measuring the temperature of the fluid flowing out of the heat exchanger and only when the device measures an abrupt temperature change, will the flow control system begin to direct the fluid to the new reservoir.
In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described by way of a non-limiting example only, with occasional reference to the annexed drawings: